1. Field of the Invention
This invention relates to battery testers and more particularly to a method and apparatus for measuring the capability of a re-chargeable battery to deliver its rated power.
2. Description of Related Art
Battery testers for lead-acid batteries have a long and complex history. There is much confusion over the purpose of such testers, how they operate and the ultimate function of the tester. Many testers merely measure the voltage delivered by a battery under load conditions. Most such testers do not even provide a constant load but merely load the battery with an element whose resistance is subject to change with environmental conditions and which produces a variable load current with change in battery voltage. The operation of lead-acid batteries in particular is complex and subject to erratic variations from a myriad of causes.
In some situations, merely determining whether a battery is fully charged is an adequate test. For many applications, particularly where batteries are used as a back-up power source for life safety equipment or have been in storage for a considerable period of time, it is not enough to know only that the battery is fully charged, for a battery may exhibit the correct voltage at its terminals, or even deliver the rated load for a short period of time, but be incapable of delivering full power over a longer period. The accepted way of testing, for example lead-acid batteries, is to place a rated load on the battery for a specified time period. For example a battery rated at 20 ampere-hours would have load of one ampere applied to it for 20 hours. The terminal voltage would be measured at the end of the period as a measure of the capability of the battery. But such a testing period is totally impractical for routine battery checks in operating systems.
Even in emergency lighting systems, it is usual to test the condition of the back-up battery supply by merely switching the light circuit to the battery supply. If the lights operate, it is assumed the battery supply is functional. This may not be true. A battery may have sufficient stored energy to provide the desired light intensity for a short period of time but be incapable of longer term operation.
U.S. Pat. No. 2,864,055 to Kordesch et al. describes a system in which a battery is periodically charged for a predetermined period and the difference in terminal voltage at the beginning and end of the charge is compared with batteries of known capacity subjected to an identical charging cycle.
U.S. Pat. No. 3,392,328 to Figg describes a battery testing method in which a storage battery is discharged for a predetermined period of time followed by a period of charging. The terminal voltage of the battery is measured immediately following the discharge cycle and again following the charging cycle. The difference between these two voltages is compared with voltages from other batteries whose power capabilities are known.
U.S. Pat. No. 3,500,167 to Appelgate et al. describes an electronic testing device in which the battery is charged and discharged in accordance with a pre-set sequence. The terminal voltage of the battery is measured at various points in the charging and discharging cycles and these values are compared with values based on batteries with known operational capabilities. The charging and discharging cycles are shorter than in the systems described above, but are relatively long. For example, the discharge cycle may be of the order of sixteen seconds followed by a rest period of some seven seconds. In other variations of the testing procedure significantly longer cyclic periods are used.
U.S. Pat. No. 3,593,099 to Scholl describes an automatic battery discharger and tester in which the test battery is discharged with a constant load and the terminal voltages continually monitored and compared with current and voltage values derived from batteries of known capability. The load is maintained on the battery until it reaches a substantially discharged condition.
U.S. Pat. No. 3,680,072 to Charbonnier et al. describes an apparatus for monitoring the charge condition of a battery. A current pulse is passed through the internal resistance of the battery and the change in terminal voltage is monitored. The device indicates when a battery has reached a minimum level of discharge and initiates a recharging cycle. The fully charged condition of the battery is determined by the voltage across the battery terminals.
U.S. Pat. No. 3,808,487 to Feuillade describes a method of charging a battery and determining the fully charged condition by measuring the results of a pulse passed through the battery that is said to measure the "operational impedance" of the battery.
U.S. Pat. No. 3,857,087 to Jones describes a number of methods for evaluating the capabilities of a storage battery. One method applies successive high current charging pulses to the battery and the current at the end of each pulse is monitored. The battery is apparently deemed to be defective if the current at the end of a second pulse is less than the current monitored at the end of the preceding pulse. Another proposed test consisted of discharging the battery through a constant resistance for five seconds, determining the battery voltage at the end of the period. A high current pulse is then passed through the battery and the "final current of the current transient" is measured. The measured voltage and the final current are compared with reference values from good batteries. Still another test is described in which a high current charging pulse is applied, measuring "the final current of current transient monitored" then discharging the battery for five seconds through a fixed resistance, delaying thirty seconds and then applying a high current charging pulse and measuring the "final current of the second current transient monitored". Another procedure consists of applying a "high current transient monitored), delaying five seconds and then discharging the battery for five seconds and monitoring, not the voltage, but the polarization at the end of the discharge. The final current transient and polarization are compared to reference values."
U.S. Pat. No. 4,053,824 to Dupuis et al. discloses a method of checking a number of battery cells connected in series. The battery cells are first charged and then allowed to remain dormant for a period up to ten hours. An "elementary transient open circuit voltage" is then measured for each cell and this voltage is compared with the average transient terminal voltages of all of the other cells. If the difference is greater than a predetermined amount, the cell being examined is presumed to be defective.
U.S. Pat. No. 4,080,560 to Abert describes a system for checking the condition of a battery by measuring (a) the no-load terminal voltage, (b) measuring the terminal voltage about 15 seconds after a heavy current load is connected to the battery, and (c) removing the current load and about 100 msec thereafter measuring a first recovery voltage, and (d) one second later again measuring the recovery voltage. A "qualitative" comparison of these measurements with predetermined reference voltages is made with "a logical evaluation" of the comparison results.
U.S. Pat. No. 4,180,770 to Eby describes a system for making certain measurements on storage batteries in which the value of the initial open circuit terminal voltage is stored in a comparison device. Thereafter, throughout the discharge period, the open circuit voltages are determine by measuring "loaded voltages with reference to the stored open circuit voltages to detect the discharge rates." The rates of discharge are used to reset the stored value of the initial open circuit voltage measurement to produce a moving reference. It is stated that the battery capacity is measured by the "memory output value". During the charge period, the "charge voltages" are measured with reference to the stored open circuit voltages to detect charge rates.
U.S. Pat. No. 4,361,809 to Bil et al. describes a battery quality determination in which the results are said to be substantially independent of battery size and temperature. The diagnostic cycle includes a discharge cycle followed by five charging pulses and a second discharge cycle. Various voltages derived during this cycle are compared with values obtained from batteries of known capacity.
These references serve to illustrate the confused and variable approaches that have been taken in an effort to provide a practical method of measuring the quality of a rechargeable battery. None of these procedures, however, has served to displace the standard accepted method of testing which is to apply a rated load to the battery for many hours.